This invention relates to a method for using a radar-like device in production wells to detect the oil/water contact in a reservoir rock.
More specifically the invention comprises a method for using a transmitter antenna for electromagnetic waves which is fixedly arranged near a production tubing inside a geological formation, and receiver antennas which also are fixedly arranged near the production tubing, preferably by cement fixation in the well. This method and the application of the radar-like device may enable the user to detect reflectors constituted by electrically conducting surfaces inside the reservoir. Such a surface of particular importance is the oil/water contact, with the water front in most instances constituting a relatively sharp transition between oil filled sand with high resistivity, to water filled sand with low resistivity thereby constituting a reflector.
Borehole logging tools utilizing the radar principle is known from U.S. Pat. Nos. 4,670,717, 4,814,768, 4,297,699, 4,430,653 and GB 2 030 414. Some of these patent use methods where it is necessary to estimate a wave propagation speed in order to be able to interpret the radar signals.
Schlumberger""s U.S. Pat. No. 5,530,359, xe2x80x9cBorehole logging tools and methods using reflected electromagnetic signalsxe2x80x9d, describe a logging tool with pulsed radar signals being transmitted from a transmitter antenna in a separate vertical section. The logging tool is freely hanging in the borehole in a cable or in a coiled tubing. Linear antenna elements are applied, being arranged parallelly with the long axis z of the tool. Electromagnetic pulses are emitted with a center frequency of 40 MHz and a highest frequency component of 120 MHz. This pulse is emitted in all directions into the formation and reflected from structures in the formation back to the tool in the borehole. The transit time of the pulse out to the structure and back to the tool is used for determining the distance between the reflecting structure and the borehole. Directional information is obtained by the fact that receiver antennas are arranged around the entire circumference of the tool, so that one may find the direction to the reflecting structure by making differences between the reflected signals. These differences may be calculated by means of electronic circuits, or subtraction may be performed by directly differentially coupled receiver antennas. One method to calculate the reflected signals"" direction is given. A disadvantage of Schlumberger""s U.S. Pat. No. 5,530,359 is that the instrument applies pulsed electromagnetic waves. This entails a spread of the frequency components already in the emitted signal, and thus the emitted signal pulse exhibits a continuously varying group velocity. The reflected signal becomes smeared out and one gets an unclear image of the reflecting structures. Near reflecting structures will also dominate over more remote reflecting structures, so that the more remote structures barely can be detected if the nearer rocks have relatively high conductivity/low resistivity. Another disadvantage of Schlumberger""s instrument is that it is not fixedly arranged by the geological formation, so that there is no provision for tracing changes in electrical parameters in the formation during a period of time, e.g. from one date to another. The instrument is also not arranged for being applied neither in production wells nor in injection wells.
Another apparatus is described in U.S. Pat. No. 5,552,786: xe2x80x9cMethod and apparatus for logging underground formations using radarxe2x80x9d, (Schlumberger). U.S. Pat. No. 5,552,786 describes a logging tool which partially solves the problem of the electromagnetic wave propagation speed in the formations which are to be logged. The apparatus emits an electromagnetic pulse in close contact with the borehole wall, into the formation and receives the direct wave in a predetermined distance along the drillstring from the transmitter. Thereby the wave propagation speed for the xe2x80x9cdirect wavexe2x80x9d through the rocks (which may be invaded by drilling mud), and the reflectors separations from the emitter/receiver system may be calculated more exactly than if one had only an estimate of the wave propagation speed.
U.S. Pat. No. 4,504,833 xe2x80x9csynthetic pulse radar system and methodxe2x80x9d concerns a synthetically pulsed radar which generates several signals of different frequencies simultaneously. The response from the subsurface to these different frequencies simulates parts of the Fourier spectrum which would have been measured if one emitted a very short pulse which according to the mathematical background would have been very broad in the frequency spectrum. However, the system is arranged to be used onboard a vehicle, among other things, because according to its claim 1, it shall be able to generate all the component signals simultaneously.
U.S. Pat. No. 4,275,787 xe2x80x9cMethod for monitoring subsurface combustion and gasification processes in coal seamsxe2x80x9d describes a radar for detecting a combustion front in a geological formation, e.g. a coal bearing formation. Due to the resistivity generally increasing with the temperature, such a combustion front will provide high resistivity and constitute a very large contrast in relation to the coal bearing formation which normally will show low resistivity.
The attenuation exceeds loo dB/wavelength in the combustion front, and the attenuation of xe2x80x9cPittsburgh coalxe2x80x9d is 1 dB/wavelength, for xe2x80x9cBritish coalxe2x80x9d the attenuation is 3 dB/wavelength. The applicant (of U.S. Pat. No. 4,275,787) mentions that a detection range for the combustion front is 100 m, an unrealistically long distance when one takes into consideration the conditions in an oil well where the attenuation of the signal is much higher and where it is a very difficult task to detect reflecting surfaces only one to two metres out in the reservoir. A swept signal is emitted and which varies continuously between a lowest and a highest frequency. Because the combustion front is moving, one may by subtraction of the received signals be able to see a change in the difference signal between the observations. However the patent does not take into consideration the need for tuning of the transmitter antennas when the transmitter antennas are lying very close (e.g. within a few millimeters) to a metallic pipe surface (e.g. liner pipe or completion pipe) and the frequency of the emitted signal is changed.
The methods and tools in the known art do not solve the problems arising in the context of petroleum production on the Troll oilfield in the North Sea as described herein, data from the Troll oil field indicate that the resistivities in the actual geological formations are relatively lower with respect to the conditions described in the known art, and therefore it is impractical to perform detection by means of electromagnetic waves by means of the known art.
A preferred embodiment of the present invention comprises a method for detecting electrical properties in a geological formation via a well which has within it a tubing string, comprising the steps of mounting a transmitter antenna outside the tubing string in the well in a fixed position with respect to the geological formation, mounting a receiving antenna outside the tubing string in a fixed position with respect to the geological formation, generating a first series of electrical signals to cause the transmitter antenna to emit a first series of electromagnetic waves at a first time, receiving a first series of reflected electromagnetic waves in the receiving antenna, transforming the first series of reflective electromagnetic waves into a first registration, generating a second series of electrical signals to cause the transmitter antenna to emit a second series of electromagnetic waves, receiving a second series of reflective electromagnetic waves in the receiver antenna, transforming the second series of reflective electromagnetic waves into a second registration. An alternative to this embodiment of the present invention comprises forming a difference between the first and second registrations to indicate a change in electrical properties between the first and second times. A further alternative to this embodiment comprises interpreting the change in electrical properties as representing a change or a displacement of a liquid horizon.
Another preferred embodiment of the present invention comprises a device for detecting electrical properties in a geological formation via a well containing a tubing string, comprising a transmitter antenna for emitting electromagnetic waves configured to be positioned on or near the tubing string and mounted in a fixed position with respect to the geological formation, and a receiver antenna for receiving reflected electromagnetic waves configured to be positioned on or near the tubing string and mounted in a fixed position with respect to the geological formation. In an alternative to this embodiment, the receiver antenna is a directionally sensitive antenna group comprising three or more receiver antennas configured to be positioned about the tubing string center line axis at a position along the length of the tubing string, and capable of detecting the direction of reflected electromagnetic waves and the direction to electromagnetic wave reflectors with respect to the tubing string axis. In a further alternative to this embodiment, the transmitter antenna comprises a transmitter antenna group comprising two or more transmitter antennas configured to be positioned about the tubing string center line axis at a position along the length of the tubing string, and capable of emitting electromagnetic waves generally in a selected direction with respect to the tubing string axis. In a further alternative to this embodiment, a transmitter antenna group is positioned between two directionally sensitive antenna groups. A further alternative to this embodiment includes an electronics package comprising a signal generator for generating electrical signals to the transmitter antennas, devices for receiving signals induced in each receiver antenna, signal processing devices for processing the received signals, and communication devices for transmitting processed signals and for receiving control signals.